Anadarko Petroleum’s board has declared Occidental Petroleum’s sweetened $38-billion takeover “superior,” giving Chevron four days to either boost its offer or walk away from the oil industry’s biggest deal in at least four years. The announcement by Anadarko, which had rebuffed several prior bids from Occidental, comes almost a month after it agreed to be bought by Chevron for $33-billion. Chevron has so far refused to increase its offer, saying its proposal is a better long-term option for Anadarko shareholders. If it decides not to raise the bid, Chevron can walk away with a $1-billion breakup fee.